LCD devices are widely used in flat panel displays for monitors, televisions and/or other displays. As is well known to those having skill in the art, an LCD display generally includes a planar array of LCD devices that act as an array of optical shutters. Transmissive LCD displays may employ fluorescent tubes above, beside and sometimes behind the array of LCD devices to provide backlighting for the display. A diffusion panel behind the LCD devices can be used to redirect and scatter the light evenly to provide uniform brightness and contrast across the display.
For example, it is known to use one or more cold cathode fluorescent tubes adjacent to one or more edges of the planar array of LCD devices, and a light guide or light pipe that directs the light from the cold cathode fluorescent tubes to illuminate the face of the planar array of LCD devices. Unfortunately, such edge lighting may be inefficient, with up to 50% or more of the light being lost.
It is also known to provide an array of cold cathode fluorescent tubes behind and facing the planar array of LCD devices. Unfortunately, an array of cold cathode fluorescent tubes may increase the thickness of the LCD display and/or increase the power consumption thereof. It also may be difficult to uniformly illuminate the planar array of LCD devices with an array of cold cathode fluorescent tubes.
Semiconductor light emitting devices, such as Light Emitting Diode (LED) devices, also may be used for edge illumination of a planar array of LCD devices. For example, U.S. patent application Ser. No. 10/898,608, filed Jul. 23, 2004, entitled Reflective Optical Elements for Semiconductor Light Emitting Devices, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, describes side emission LEDs that may be used for large area LCD and/or television backlighting.
LED arrays have also been used as direct backlights for transmissive LCD displays as described in U.S. patent application Ser. No. 11/022,332, filed Dec. 23, 2004, entitled Light Emitting Diode Arrays for Direct Backlighting of Liquid Crystal Displays, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
As is well known in the art, an LCD screen may generate a color image by providing a planar array of red, green and blue (RGB) pixels. By varying the intensity of each of the three colors, a multitude of colors may be generated by a single RGB pixel. A single color (i.e. red, green or blue) pixel 15 of an LCD display illuminated by a fluorescent light source 12 is illustrated in FIG. 1. In a fluorescent-based LCD display system 10, a fluorescent light source 12 generates a high-intensity light 14A, which is directed towards the pixel 15 as illustrated in FIG. 1. As is also well known in the art, a fluorescent light source may generate broad spectrum light that includes wavelength components in the red, green, blue and other portions of the visible spectrum. Light generated by a fluorescent light source 12 may be perceived by an observer as white or near-white light.
Light 14A emitted by the fluorescent light source 12 passes through polarizer 16, which is configured to permit only light 14B that is polarized in a predetermined first direction to pass through it. Thus, a significant amount of light may be absorbed and/or blocked by the polarizer 16. Accordingly, in the illustration of FIG. 1, the arrow that indicates polarized light 14B passing through the polarizer 16 is smaller than the arrow representing light 14A generated by the fluorescent light source 12. A transflective surface (not shown) such as a two-way mirror may be provided between the light source 12 and the polarizer 16. A transflective surface may transmit light from the light source 12 and reflect light coming in from the viewing surface back into the polarizer 16. In this way, the display can be illuminated by a backlight and/or by ambient light.
Next, the polarized light 14B that passes through the polarizer 16 passes through a liquid crystal shutter 18, which is configured to either allow the polarized light 14B to pass through the shutter 18 unchanged or to re-polarize the light 14B to a second polarization direction, based on the charge state of a pair of transparent electrodes (not shown) on either side of the liquid crystal shutter 18. Typically, the second polarization is rotated 90° from the first polarization direction. Thus, for example, the liquid crystal shutter 18 may re-polarize light passing through it when a voltage is applied to the electrodes and may allow light to pass through unchanged when no voltage is applied to the electrodes. In either case, little or no significant absorption of light may occur in the liquid crystal shutter 18. Thus, light 14C exiting the liquid crystal shutter 18 may have substantially the same intensity as light 14B entering the shutter 18. Accordingly, the arrow representing the light 14B is substantially the same size as the arrow representing the light 14C in the illustration of FIG. 1.
The formation of transparent electrodes for LCD displays is well known in the art. For example, the transparent electrodes may be simple electrodes, as in the case of a passive display, or, they may be thin film transistors (TFT) using amorphous silicon, low-temperature poly-Si (LTPS) TFTs, or organic thin film transistors (OTFT) in the case of an active display. Cadmium selenide (CdSe), or similar high mobility amorphous/low temperature deposition process material has been used as the semiconductor material in the thin film transistor as described in U.S. Pat. Nos. 5,650,637 and 5,365,079.
The light 14C exiting the liquid crystal shutter 18 then passes through an analyzer 20, which may simply be a polarizing filter that is configured to pass only light polarized in a second polarization direction and to block, for example, light polarized in the first polarization direction. In this manner, the polarizer 16, the liquid crystal shutter 18 and the analyzer 20 together function as an optical switch that selectively passes/blocks light impinging on the LCD pixel depending on the voltage of the control electrodes. When polarized light 14B from the polarizer 16 (which is polarized in the first polarization direction) is repolarized in the second polarization direction by the liquid crystal shutter 18, it may pass through the analyzer 20. In contrast, when polarized light 14B from the polarizer 16 is not repolarized in the second polarization direction by the liquid crystal shutter 18, it may be blocked/absorbed by the analyzer 20 and not permitted to pass therethrough.
Light that does pass through the analyzer 20 is then filtered by an optical bandpass filter 22, which may remove significant optical energy from the light passing therethrough so that only a narrow band of light 14D in the red, green or blue region of the visible wavelength spectrum passes through the optical filter 22. Thus, in a conventional fluorescent-based LCD system, significant optical energy may be lost in both the polarizer 16 and the filter 22. It will be appreciated that the optical filter 12 could be placed at other locations in the device. For example, the optical filter could be placed between the light source 12 and the polarizer 16.
As will be apparent from the foregoing discussion, a broad spectrum light source such as a fluorescent backlight may generate energy outside the passbands (i.e. the range of frequencies allowed to pass) of the red, green and blue optical bandpass filters 22 of the display that will never be emitted as useful light by the display. Such light essentially represents wasted energy.
Accordingly, in order for the LCD display 10 to provide a given level of light output to the user, the fluorescent backlight 12 must be capable of generating sufficient optical energy to overcome the above-described losses. Loss of optical energy may also increase the amount of heat generated by the display, which may reduce the operating lifetime of the display, in addition to other undesirable effects. For example, for a battery powered electronic device, high backlight brightness and long operating times may require a large and/or expensive battery; correspondingly, designers of portable electronic devices must make a compromise with brightness or operating time to maintain small and/or inexpensive batteries.
An LCD display system 30 having an LED-based backlight system is schematically illustrated in FIG. 2. As illustrated therein, an LED light source 26 may generate unpolarized light 24A having energy in the red, green and blue portions of the visible light spectrum using, for example, an array of red, green and blue (RGB) light emitting diodes. The unpolarized light 24A generated by the RGB LED light source 26 is passed through a polarizer 16 which, as described above, only permits light polarized in first polarization direction to pass therethrough. Thus, as with the fluorescent-based LCD display system 10, otherwise useful light may be lost in the polarizer 16 in an RGB LED-based light system 30.
The remainder of the RGB LED-based LCD display system 30 is similar to that of the fluorescent-based LCD display system 10. That is, polarized light 24B exiting the polarizer 16 passes through a liquid crystal shutter 18 which either re-polarizes light passing therethrough or permits light to pass through unchanged depending on the state of a pair of transparent electrodes (not shown) disposed on either side of the liquid crystal shutter 18. Light 24C passing through the liquid crystal shutter 18 is then either blocked or allowed to pass by an analyzer 20 which, as stated above, may be a polarizer having a second polarization direction different from the first polarization direction. An optical bandpass filter 22 then filters out wavelengths other than a desired (R, G or B) wavelength band, resulting in light 24D output by the pixel 15 having a desired wavelength.
While the bandwidths of the light output by the red, green and blue LEDs may be better matched to the passbands of the optical filters 22 of the display system 30, the passbands of the optical filters 22 may still be somewhat narrower than the bandwidths of the LED light sources 26. This is because as the passbands of the filters (i.e. the bandwidth of light passing through the filters) is reduced, the color saturation (i.e. the purity) of the light increases. When the pixels of an LCD display 30 emit more highly saturated red, green and blue light, the pixels may be capable of displaying a wider range of colors. Thus, even though an LED-based backlight 26 may produce light having more selective bandwidths, some optical energy may still be lost in the optical filters 22. However, the majority of optical absorption in an LED-based display system may occur in the polarizer 16.
Accordingly, it will be appreciated, with an RGB light source, less energy may be lost in the filters compared to a white light source. There are two possible benefits to this. The first is that in displays with comparable passbands of the filters, the power consumed by a RGB light source will be less than that of a white light source to obtain comparable display brightness (assuming equal backlight efficiencies). The second is that with an RGB back light source, the passbands of the filters could be made more narrow to improve color resolution and image purity while maintaining the same level of power consumption.
From the foregoing discussion, it is apparent that there is a tradeoff between the bandwidths of the optical filters 22 and the amount of light generated by the backlight. Narrower bandwidths for the optical filters 22 may lead to better color rendering in the LCD display. However, as the passbands of the filters 22 are reduced, less light may be emitted by the display, and more light must be generated by the backlight.
In addition to being able to generate a wide range of colors, it is also desirable for an LCD pixel to be able to generate a dark black color (for example, as opposed to a dark gray color) when the pixel is “off” by blocking substantially all light generated by the backlight. The ability of a conventional LCD display to block a sufficient amount of light to generate a dark black may be limited by the efficacy of the polarizer 16 and/or the analyzer 20. For example, there is a trade-off between the efficacy of the polarizer 20 and the amount of light it blocks. LCD manufacturers may strike a balance between the two. For example, a polarizer 16 that passes only highly polarized light may block too much light, resulting in a dim display. In order to permit more light to pass through the polarizer 16, the strength of the polarizer 16 may be reduced, which may permit some light with “stray” polarization to pass through the polarizer 16. As a result, even when the pixel is “off” and the polarization of light 14B, 24B passing through the polarizer 16 is not rotated by the liquid crystal shutter 18, some of the light 14B, 24B passing through the polarizer 16 may have a polarization that will allow it to pass through the analyzer 20. This may result in some light 14D, 24D being emitted by the pixel 15 even though it is in the “off” state.
In addition, the strength of the analyzer 20 may be balanced against the amount of light 14A, 24A generated by the light source 12, 26. The strength of the analyzer 20 may be related to its thickness. Thus, a thicker analyzer may more effectively reject improperly polarized light. If the analyzer 20 is made thin in order to permit more light to pass through it, it may not reject light that should otherwise be blocked, for example when the pixel 15 is in the “off” state, which may make the display of dark black pixels difficult.